Hello,
Try to increase the PHP Max Input Vars parameter to 10000 or higher in your PHP configuration. You can contact your hosting provider for help on this matter and check how it works.
If the issue remains, please deactivate all the 3rd party plugins and activate only theme-required plugins on the site, and then check the issue. I am sure your issue will be solved. Then activate the 3rd party plugins one by one and clear the server cache, and check which plugin is creating the issue for you.
If the issue still remains, let me know and I will check and give you a possible solution.
Best Regards,